1. Field of the Invention
The present invention relates generally to a fuel supply control system for an internal combustion engine, such as an automotive internal combustion engine. More specifically, the invention relates to a fuel supply control system which can achieve improved transition characteristics in a transition from an engine deceleration state to an engine acceleration state.
2. Description of the Background Art
In the modern and advanced automotive technologies, it has been required substantially high level of precision in engine operation control in view of anti-polution, high performance and better fuel economy. One of the essential factors in achieving high precision level engine operation control controlling fuel supply amount with satisfactorily high precision. Particularly, in the recent years, it has been required to correct fuel supply amount for compensating fuel wetting an air induction passage and thus cannot be introduced into an engine combustion chamber at a desired timing.
The conventional fuel supply control system controls fuel supply amount typically based on an engine revolution speed and an intake air flow rate which is monitored by an air flow meter disposed in the air induction system upstream of a throttle valve. Such conventional fuel supply control system has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 59-538. The conventional fuel supply control system cannot achieve required level of precision because of distance between the air flow meter and a fuel injection valve. Namely, since the air flow meter is disposed at a position upstream of the throttle valve and the fuel injection valve is disposed at a position downstream of the throttle valve, the intake air flow rate at the position of the air induction passage where the fuel injection valve is disposed, is normally different from that at the position of the air flow meter. This particularly affects upon accelerating transition of the engine. Furthermore, fuel injection amount is derived irrespective of the fuel amount which wets the inner periphery of the air induction passage. Therefore, air/fuel ratio tends to become lean to cause degradation of the engine acceleration characteristics.
In order to improve this, Japanese Patent First Publication (Tokkai) Showa 60-162066 owned by the common owner to the present invention, proposes a fuel supply control system, in which intake air flow rate is derived by smoothing the output of the air flow meter with a primary lag factor and the fuel injection amount is derived on the basis of the smoothed intake air flow rate. Utilizing of the smoothed air flow rate data for deriving fuel injection amount encounters a defect in that the fuel injection amount becomes smaller than required amount at initial stage of acceleration transition to make the air/fuel ratio of the air/fuel mixture unacceptably leaner than that required. In addition, in the transition from the engine decelerating state where fuel cut-off is performed to the engine acceleration state in which fuel supply is resumed. In the fuel injection control disclosed in the aforementioned Japanese Patent First Publication Showa 63-162066, asynchronous injection irrespectively engine revolution cycle is performed for acceleration fuel enrichment at the initial stage of acceleration. Fuel injection amount for asynchronous injection is derived based on a fuel injection amount upon initiation of fuel cut-off operation and a fuel injection amount derived on the basis of instantaneous fuel injection control parameters including the smoothed air flow rate. During fuel cut-off state, fuel on the peripheral surface of the air induction passage is drawn into the engine combustion chamber to dry the periphery. Therefore, upon resumption of fuel injection, relatively large amount is required for wetting the periphery of the air induction passage.
In the aforementioned conventional fuel supply control system, fuel amount for asynchronous injection for acceleration enrichment simply based on the difference between the fuel injection amount upon fuel cut-off and upon fuel resumption. Therefore, certain amount of fuel injected in asynchronous injection is consumed for wetting the periphery of the air induction passage. This makes the amount of fuel actually forming air/fuel mixture to be introduced into the engine combustion chamber becomes insufficient for obtaining desired engine acceleration characteristics.